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Last updated: 6 Sep 2023

Fall back to AWS CloudFront

There is a backup Content Delivery Network (CDN) that can be used if Fastly is down. This backup CDN is currently provided by AWS CloudFront.

Important The failover CloudFront distribution does not have feature parity with the primary Fastly service. Most features will continue to work, including search, smart answers, and postcode lookups. Some features, including A/B testing, will not work.

Fail over checklist

The last time we needed to initiate a CDN failover, we found that Terraform does not work reliably when Fastly is having a major incident. The situation might have changed since then, but it is still recommended to perform the failover manually, before attempting to update our Terraform configuration.

  • Confirm that Fastly is the cause of the incident (check https://status.fastly.com/ and keep an eye on twitter - if there’s a major Fastly outage there will be a lot of noise)
  • Escalate to GOV.UK SMT as soon as you begin to consider failing over
  • Sign in to the AWS console as an admin (gds aws govuk-production-admin -l, or however you prefer to sign in to AWS)
  • Sign in to the GCP console
  • For production, open the following four pages as separate tabs:
  • For staging, open the following three pages as separate tabs:

  • You are going to update the CNAME records for two different domains, in both GCP and AWS:

    • For production, these two domains are www-cdn.production.govuk.service.gov.uk and assets.publishing.service.gov.uk
    • This Draft PR to Failover to AWS CloudFront shows the CNAMEs you need to change, and how to test that they are correct
    • You can also get the CNAMEs to use for the secondary CDN from the AWS CLI:
      # www-cdn.production.govuk.service.gov.uk
  gds aws govuk-production-readonly aws cloudfront list-distributions --query "DistributionList.Items[?Aliases.Items[0]=='www.gov.uk'].DomainName | [0]"
  # assets.publishing.service.gov.uk
  gds aws govuk-production-readonly aws cloudfront list-distributions --query "DistributionList.Items[?Aliases.Items[0]=='assets.publishing.service.gov.uk'].DomainName | [0]"
    • For staging, these two domains are www.staging.publishing.service.gov.uk and assets.staging.publishing.service.gov.uk
    • You can get the CNAMEs to use for the secondary CDN from the AWS CLI:
      # www.staging.publishing.service.gov.uk
  gds aws govuk-staging-readonly aws cloudfront list-distributions --query "DistributionList.Items[?Aliases.Items[0]=='www.staging.publishing.service.gov.uk'].DomainName | [0]"
  # assets.staging.publishing.service.gov.uk
  gds aws govuk-staging-readonly aws cloudfront list-distributions --query "DistributionList.Items[?Aliases.Items[0]=='assets.staging.publishing.service.gov.uk'].DomainName | [0]"
    • In either case, the records should look like d0000000000000.cloudfront.net. (with 0s replaced with letters and numbers)

  • Manually update the CNAME records for both domains in both GCP and AWS, via the tabs you opened in your web browser earlier

  • On production only, after performing the manual failover, you should also update our infrastructure-as-code to match the changes you just made:

  • Once you’ve failed over, keep a close eye on Fastly’s status

  • As soon as you are confident that Fastly has recovered

    • Manually set each of the CNAME records you changed above back to www-gov-uk.map.fastly.net
    • If you previously raised a PR in govuk-dns-tf, raise another PR to revert your changes and restore the old records. Get it approved, merged and approve the Terraform apply via the govuk-dns-tf workspace on Terraform Cloud.

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